The science behind improving your gut health

KHP comms officer Tom Smurthwaite sat down with Prof Kevin Whelan, Dr Eirini Dimidi and Dr Balazs Bajka from the Department of Nutritional Sciences at King’s College London to discuss the relationship between diet and health.

The conversation ranged from the relationship between gut microbiomes and the body’s other organs, and how diversity is key when it comes to getting more fibre in our diets.

Please explain how the gut works

Dr Balazs Bajka [pictured right]- The gut is an extremely complicated organ whose basic function is to disassemble the food we eat to enable the absorption of the nutrients our body needs to function. It consists of multiple regions and multiple mechanisms, from the mechanical breakdown of food like chewing and muscle contraction, to the chemical digestion of foods, such as digestive enzymes breaking down the fat, protein and carbohydrates that we eat into a form that our body can then absorb.

The breakdown occurs initially in the mouth and the stomach, and the absorption primarily occurs in the small intestine. As the food moves down the small intestine and the body extracts the maximum amount of nutrients that it can, it then enters the large bowel where there are more physiological processes from the body. The main thing that occurs here is fermentation from the microbiome.

Prof Kevin Whelan [pictured right]: We used to think that the gut microbiome - the bacteria that live in your gut - just sit in the gut chomping away at the leftovers after our body had digested the rest of our food and that they didn't really do much else. But in the last 20 years we’ve realised the gut microbiome have really important functions.

They help protect us from infection and disease by competing with other bacteria that may be trying to infect our bowel. They reduce the pH of the bowel to make it more acidic so that it's not as easy for harmful bacteria to live there. They also help to stimulate our immune system and help us differentiate between harmful and non-harmful bacteria.

For example, if we get a salmonella infection they prompt the immune system to help us attack the infection. Importantly, the microbiome also teaches our immune system to be tolerant to the friendly bacteria in the bowel.

Overall, the microbiome is really important in maintaining our gut health, and overall health.

Leading on from that, how important is gut health in a person’s overall health?

Dr Eirini Dimidi [pictured right]: I echo everything that Kevin has already said about the gut microbiome in protecting, maintaining and promoting health. Another way to look at it is what happens when gut health is not optimal and not well-maintained.

There are certain conditions that affect gut health, such as irritable bowel syndrome, constipation or inflammatory bowel disease. We know from research and patient feedback that this has a significant impact on a person’s quality of life and day-to-day activities, and also their mental health.

The microbiome doesn't only affect gut health and gut function, we know that it can also affect what happens to other organs in our body. For example, we have what we call the gut-brain axis, which is the communication that takes place between the gut and the brain.

We know that what happens in the gut, through the gut microbiome, can affect brain function and mental health, we've seen that in both animal research and human studies. So there are many different reasons why gut health is important.

What is the best diet for good gut health?

Prof Whelan: Firstly, it's important to understand that there is no one diet. Everybody eats completely differently – even people who live in the same house eat differently to each other – and we should really embrace that diversity. When you're thinking about gut health, it's not about me telling you exactly what you should eat for breakfast, lunch and dinner. It's more about describing the principles of what you should include in your diet and working out how you can achieve that doing it your way – you do you!

For good gut health we encourage a high fibre intake. In the UK we recommend adults eat 30 g of fibre per day, but on average we’re eating much less than that - between 18 g and 22 g per day - so we need to increase our fibre intake.

From the work we've done in our department we know it's not just about eating 30 g per day, it's about eating different types of fibres. So making sure that you're getting different fibres from different sources: fruits; vegetables; cereals; nuts; seeds; and other high fibre containing foods.

We know that diversity is important. Even if you eat lots of fruit and veg, and get your five-a-day, it's important that you switch those around. Different foods all have different nutrients in them that are important for the gut, so make sure you're not just having the same meal every day, no matter how much fibre it may contain.

Dr Bajka: It's important from a fibre perspective to talk about diversity. We talk about fibre as a single entity, but it's not. It's a vast array of different chemical and physical structures that all have different potential benefits, both in the upper gut but they are also all fermented differently and have different outcomes.

Dr Dimidi: In terms of fruit and vegetables and diversity, and getting fibre from different sources, its not just fibre that is important, it is also polyphenols. They’re another type of compound that we know impact gut microbiome and health, and are fermented by our gut microbiome.

Polyphenols are what gives fruits and vegetables their colour, so going for different colours of fruits and vegetables can ensure we get many different types of polyphenols, as well as different types of fibre, to help maintain gut health.

Prof Whelan: Another thing to consider is seeking ways to get live bacteria into our diet, things like probiotics and fermented foods. You can think about taking them from probiotic products like fermented milks and probiotic yoghurts, or from fermented foods like kombucha, kimchi and miso.

There are not many studies currently available showing that fermented foods are effective in modifying the microbiome in your gut, we are currently working on this, but we believe that they may be beneficial in supplementing live bacteria to the diet in terms of overall health.

Dr Bajka: Additionally, the structure of the food we eat is important in our diet. The rate at which the gut can break food down is important in terms of how the body then metabolises the nutrients. Food structure is currently quite a common topic, and has importance in terms of the absorption that happens directly after a meal, before the food gets to the large bowel. This can be as simple as differences in particle size, as our department’s research on course vs fine ground wheat flour has shown.

This is important for people with diabetes, where you want to restrict the glucose excursions after a meal and have it released over a longer period but at a lower level. It can also affect satiety and the hunger after a meal, so it's important for conditions like obesity when eating food that’s digested more slowly is beneficial. There are a lot of underlying physiological studies that support this.

What should people avoid?

Prof Whelan: There is a lot of discussion in the media about ultra processed foods (UPF) and how these may be harmful for our health, particularly gut health. But it's important that scientists help the public to distinguish between observational studies and intervention studies.

Almost all of the studies of UPF are observational, where we watch what people eat over many years and then see whether they get any diseases. It doesn't tell us that ultra processed foods cause poor gut health, it shows there is an association. What we need are intervention studies to be able to find out their true cause and effects of UPF on health and disease.

By UPF we’re talking about things like ready meals, supermarket breads, crisps and dairy desserts and anything with food additives. However, there are anomalies in how UPF are defined. For example, supermarket wholemeal bread, fortified fruit juices and plant milks are categorised as ultra processed but can be healthy for our diet.

I'm not in any way trying to say UPF are good for us, but I’d prefer to focus on the health benefits of improving your diet rather than demonising certain foods. I would encourage people to buy whole foods, fresh fruits and vegetables, and to make diverse meals and cooking from scratch. I think that's a healthy way of trying to interpret the ultra processed food debate, rather than demonising them.

Dr Bajka: There's evidence out there that antibiotics can have deleterious effects on the microbiome, although we don't want to suggest that people don't take prescriptions that their doctors have given them.

Prof Whelan: Yes, that’s right. We know that antibiotics do change the gut microbiome and in some people it rebounds and normalises, but in other people it doesn't normalise. So we would encourage the sensible use of antibiotics - if your doctor recommends that you take antibiotics, then you should follow their advice. But we shouldn't be trying to take more antibiotics than we need.

What are the signs of an unhealthy gut?

Dr Dimidi: The obvious first one is if someone experiences gut symptoms, for example regular and unusual abdominal pain and bloating. This may be a sign there's something going on that’s worth investigating. Also, an increase or decrease in the frequency of your bowel movements, or a change in the consistency of your bowel movements, either harder or looser, may be a sign of your gut health changing.

When we talk about changes in stool frequency or stool consistency it's important to mention that this can happen in a healthy gut and it's not necessarily that there's something serious going on. They can be sporadic, but it's more when it's a chronic change that perhaps it's worth looking at.

Prof Whelan: Yes that’s right, everyone's bowel habits are different, but you should always monitor them to see if there’s a huge change.

There may be small changes in response to things like diet or travel that are perfectly normal and you don’t need to worry about. But if you have a dramatic change in stool frequency and stool consistency, that happens over a longer period, then that may indicate a change in your gut health.

Dr Dimidi: However, the presence of blood in your stool is a red flag and you should see your GP. Also, any unexpected changes in body weight may also be a sign of an unhealthy gut. These are signs that are worth discussing further with your doctor.

Prof Whelan: Also, lots of people are paying a fortune to get expensive stool microbiome testing, it’s happening all over the world. While this may be of great personal interest, there is insufficient scientific evidence that doctors and dietitians can use this information to make recommendations on what to eat based upon the self-testing of your gut microbiome.

At this stage, we don’t recommend online/postal stool microbiome testing except for personal interest. If we get this scientific advance, I will let you know!

And one important thing, it’s quite normal to pass wind. It is usually a sign that your microbiome is feeding on the fibre in your gut. Although it might have olfactory consequences for those around you, it is only a problem if it becomes excessive.

What is an easy quick win for good gut health?

Prof Whelan: With the current cost of living crisis and some people struggling to eat well it can be difficult to buy the specialist products sold on the market for your gut health, but in general you don't need to do that. There are plenty of easy and cheap quick wins for your gut.

A large meta-analysis of observational studies of fibre and its association with health showed that an extra 7 g of fibre per day was associated with between a 7% and 9% reduction in heart disease, colon cancer and diabetes. 

There are some really quick, cheap and easy ways to get an extra 7 g of fibre in your diet.

For example, a baked jacket potato with broccoli will add 7 g of extra fibre to your diet, as will half a tin of baked beans.

Dr Dimidi: People tend to have about two snacks a day, so snacks are a great opportunity to add fibre to your diet. One way we can do that is by eating a variety of different nuts and seeds, so we get a range of different fibres and other nutrients into our diet. We have conducted research in the Department of Nutritional Sciences showing that increasing the intake of nuts is associated with improved bowel health.

Dr Bajka: Kevin mentioned that an increase in fibre has a benefit beyond what happens in the gut. In terms of the cardiovascular system β-glucan (beta glucan), a specific type of fibre contained in oats, has been demonstrated to have significant benefits in terms of lowering plasma cholesterol levels.

Increasing the diversity of the types of cereals and grains that you consume can be beneficial in specific ways, and eating oats for breakfast is a fantastic way of doing that.

Another benefit in diversifying sources of fibre and carbohydrates is increasing consumption of pulses and legumes. They're not only a great source of fibre, but also protein and slow-release digestible carbohydrates. This is important for maintaining blood glucose within reasonable normal levels.

This comes down to the structure. So pulses in particular, the fibre is in the cell walls. Our research has shown this forms a natural capsule that changes the rate at which nutrients can then be absorbed, and they are highly fermentable in the large bowel and help to keep us feeling full. They are an excellent source of dietary fibre.

To learn more about the Department of Nutritional Sciences, visit its website here.

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Prof Kevin Whelan is the Professor of Dietetics and Head of Department of Nutritional Sciences at King’s College London. His research investigates the interaction between diet and the gut microbiome in health and in gastrointestinal disorders such as inflammatory bowel disease, constipation and irritable bowel syndrome. He has published over 200 journal articles on fibre, probiotics, prebiotics, the low FODMAP diet and food additives and their impact on the microbiome in health and disease. Kevin is a Founding Trustee of the Academy of Nutrition Sciences, and a Fellow of the British Dietetic Association.

Dr Eirini Dimidi is a Senior Lecturer at the Department of Nutritional Sciences, King’s College London. Dr Dimidi is leading research on nutrition-based interventions, including fibre, prebiotics, probiotics, plant foods, and the low FODMAP diet, in gut function and dysfunction (e.g. constipation). She is also investigating the mechanisms through which nutritional interventions affect immune and mental health via the gut microbiome. Other research interests include exploring patients’ experiences and perceptions of gut diseases. She has published in a number of peer-reviewed nutrition and gastroenterology journals and has been awarded the Nutrition Society Cuthbertson Medal 2023 for her research output.

Dr Balazs Bajka is a Lecturer in the Department of Nutritional Sciences, King’s College London. As a gut physiologist, his research investigates the role of food structure on digestion kinetics, postprandial glycaemia, lipaemia and gastrointestinal endocrine responses. Of particular interest is how dietary components, like dietary fibres, can alter nutrient bioavailability. His ongoing research demonstrates that the macro-molecular structure of dietary fibre (plant cell walls) can regulate starch digestion kinetics and reduce lipid diffusion through intestinal mucus with the aim of developing better strategies for the prevention and treatment of conditions such as obesity and type-2 diabetes.